So-called SCR (selective catalytic reduction) systems are used for reducing nitrogen oxides (NOx) from the exhaust gases of a diesel engine. Nitrogen oxides are thus converted, by means of a catalyst, to nitrogen gas (N2) and water. A gas reductant, e.g. anhydrous ammonia, water-dissolved ammonia or urea, e.g. AdBlue, is added to a flow of flue gases or exhaust gases and is absorbed by a catalyst. Carbon dioxide is a reaction product when urea is used as reductant.
Using the SCR system involves injecting, for example, AdBlue at high pressure into the flue gases by means of an injector. A regulator is used to regulate the pressure in a high-pressure hose connected to the injector. The system regulated comprises a pump, hoses and an injector with a pressure sensor. The amount of gas reductant (AdBlue) added depends inter alia on measured contents of nitrogen oxides, preferably measured downstream from where the injector is situated. The gas reductant is added by opening and closing the injector, the amount of gas reductant being controlled by the open time for the injector. A typical cycle time for the injector, i.e. the period between two consecutive openings of the injector, is preferably of the order of 0.5 to 1.0 second.
The regulating parameters of the regulator depend inter alia on the characteristics of the high-pressure hose, such as its length and diameter and the softness of its material, e.g. rubber. The regulating parameters are also affected by temperature.
There are thus several characteristics which interact here and are covered, from a regulating perspective, by the concept of hose rigidity. A difficulty is that the hose rigidity is unknown, e.g. at the time of installation, and therefore needs somehow to be determined. One way of doing this is to manually calibrate the rigidity of every type of hose which might be relevant to the respective installation. However, this is very time-consuming and involves problems, e.g. if hoses have to be changed.
The object of the present invention is to simplify and improve the determination of hose rigidity so as not only to lead to less expensive handling but also afford advantages pertaining to hose changing in that no separate calibration need be done for each fresh hose.